Where can I go to get runes? Whats the best place to go? Who drops what?
Countess is a widely known critter for dropping runes. She always has the best chance at getting a rune to drop. She is not 100% garunteed to drop one, just the best place to get one. She has an interesting drop table as she has what is known as a special rune drop. It is best to hunt the countess in solo games if you are after runes. If you are after Terror keys, more partied members the merrier. Ideally we want her normal drop to include as many "Nodrop" picks as possible because these will pass over to her special rune drop. Since we do know that MF% plays no part what so ever in getting a rune to drop, the best character to farm her with would be a very fast killer. My suggestion would be a lightning sorc focused on damage and if you partake in duped runes, have an act2 merc weilding an infinity. The merc's infinity will eliminate any lightning immunes in the tower and allow you to kill anything around.
The countess' special drop in hell allows for runes up to IST. She herself can drop from her normal drop up to LO and her minions can drop even higher. The odds to get an IST from the countess in hell in a solo game are 1 in 1000 or so. This is the best place in the game to get any rune IST and down to HEL. If you want something lower than HEL, do NM countess.
So what about above IST? The next best place to get runes above IST, aside from the few you can garner from the hellforge, hell travanical is the next place to go. It a decent spot to go if you want to get runes from IST to LO. There are plenty of unique monsters in travanical and they all drop nice items. For runes SUR and above, the next best place to go is hell cows. While it is true that Hephasto, river of flame next to forge, has the best drop odds in the game for these runes, why waste 2 minues and 1 game for one kill? Make the cow portal and make 200 kills in 2 minues with very little drop in the odds for what you are after to drop.
Special mention for the BER rune:
It is a known oddity that the super chests in lower kurast in hell in a full game have a "bugged" drop. While these superchests should have well over 1 million different drop possibilites, in a full game, they only have about 64,000 drop posibilites. It has been tested on other forums that this is true to the point that people swear by it. These superchests exist in the 2 huts that suround the campfire. Some maps have 1 campfire while others have 2. While it is more time efficent to run these chests in single player where the map does not change and you can always have full games, it is possible on b.net to do these chests during a baal run. Before you make your way to lower kurast, wait for the game to be full. Then take the wp to LK and tele around until you find the campfire. The hut to the north of the fire has a weapon rack and 1 super chest and the hut to the west has 2 superchests. Directly north the hut to the west is an armor rack. It is worth clicking on all 5 of these popables. If you have time, search for a second campfire. Maybe have some one call out the waves to let you know how long you have. If you are in a full game and you pop a chest and a BER comes out, you will also get from that chest an arbalast, gothic plate, arrows, gold, pots. The quality of the equipment could be magical or rare and the stats will always be random. Other runes known to pop out on a regular basis from these chests are VEX, MAL, and PUL.
Special mention for the Wraith creature:
These creatures have a wierd drop table in that they favor runes slightly. These things can be found in the tower as well as other places in the game. Most famously in the arcane sanctuary. While it is rather dificult to kill these things over the pathway in arcane, this is one of the best spots to get runes VEX to LO. I do not recomend an "area" killer to hunt here as you will make kills when these things are not over the pathway. Some sort of elemental mele character would be best here as these things can be physical immune in hell. The key when killing these things in arcane is to kill them over the path. Who knows how many runes have fallen over into the abyss when these things die not over a pathway.
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Since I am at work now, I do not have access to the exact drop odds for these runes from these critters. Once I get a chance, i will update this. Any questions, post here and do not PM me please.